Super absorbent polymers (SAPs) are the principle absorbent materials used to absorb physiological solutions in diapers, feminine hygiene and adult incontinence products. The majority of SAPs in commercial use are particles consisting of polyacrylate polymers derived from acrylic acid, which is made from petroleum. Polyacrylate based SAPs have outstanding absorbency properties, the most important of which are Free Swell Capacity (FSC), which is a measure of the grams of a standardized solution that can absorbed per gram of particles. Centrifuge Retention Capacity (CRC), which is a measure of how many grams of solution remain absorbed by the particles when placed under centrifugal force under standardized conditions of force and time, and Absorbance Under Load (AUL), which is a measure of the grains of a standardized solution that can be absorbed per gram of particle while the particles are subject to a standardized compression, load (typically 0.7 Psi), which mimics the effects of a person's weight on the absorbent product. The standardized solution used to measure these properties is 0.9% saline (NaCl), which has an osmolarity that mimics physiological solutions such as urine and blood. To be a commercially effective SAP, the particles should demonstrate minimum values of about 30-40, 25 and 15-18 in g/g for FSC, CRC and AUL, respectively, for the saline solution.
Another important absorbency property of SAPs is the rate at which a standardized amount of the standardized solution can permeate through a standardized amount of the SAPs in a standardized test. Various manufacturers use different tests for this fluid How property and may refer to it as Gel Bed Permeability (GBP) or Saline Flow Conductivity (SFC). In the description that follows a similar test called Saline Flow Rate (SFR) is used to measure the flow of 0.9% saline in a standardized apparatus filled with test SAPs. Permeability is important for SAPs because even though particles may have outstanding FSC, CRC, and AUL properties, the rapid swelling of the initially contacted particles can cause gel block, which is a damming effect that, slows the flow of the solution to subsequent particles.
Polyacrylate derived SAPs have two disadvantages from an ecological perspective. First, they are derived from non-renewable petroleum resources. Second, they are not biodegradable because few, if any natural soil organisms can digest the polyacrylate backbone of the polymer. Accordingly, efforts have been made to find ways to utilize natural polysaccharide polymers as outright substitutes or as amendments for conventional SAP to reduce the amount of polyacrylates used in the products. The most useful polysaccharides in this regard are those that have a backbone substituted with ionic (polar) substituents, such as alkylcarboxylate or amine groups. Amine groups occur naturally in some polysaccharides, such as chitosan, while carboxylate groups must be artificially added to the most abundant polysaccharides, cellulose and starch, using carboxy alkyl donating reagents. The most common carboxyalkylated polysaccharides are carboxymethyl cellulose (CMC) and carboxymethyl starch (CMS). Several attempts have been described to use such polysaccharides as outright substitutes for polyacrylates SAPs or as grafts or copolymers that may be used in conjunction with conventional SAPs.
U.S. Pat. No. 4,116,899 discloses compositions made from hydrolyzed polyacrylonitrile (H-PAN) mixed with starch. The gels formed of the combination of these materials blended together were dried and heated. It was discovered that starch and H-PAN reacted together upon heating resulting in a composition that had increased water absorbency over either alone. The reaction would likely form cross links between the starch and the H-PAN by inter esterification between the starch molecules and H-PAN. No surface treatment or other cross linking of the gels is disclosed nor proposed.
U.S. Pat. No. 5,409,771 discloses cyclic carbonate surface treatment of conventional polyacrylate SAPs and mentioned the possibility of incorporating ordinary starch with the polyacrylates. However, there is no teaching of actual production of such a combination or the properties that might be obtained thereby.
U.S. Pat. No. 4,483,950 describes use of modified starches as extenders for polyacrylate based SAPs. It was discovered that adding white dextrin to polyacrylate gels increases their absorptive performance. In fact, the hybrid SAP was a starch-graft-H-PAN made by saponifying the polyacrylate and blending it with white dextrin in solution. The blended materials formed gels with increased FSC by up to 14%. No surface treatment or any other cross linking of the gels is disclosed.
U.S. Pat. No. 4,883,478 discloses SAP formulations combined with mono or oligosaccharides. Polyacrylate SAPs were mixed, in the gel form, with saturated solutions of sucrose or corn syrup, which is a blend of glucose, some maltose and low molecular weight oligosaccharides. The amounts of sucrose in the polyacrylate blend mixture was quite high, reaching even 90%. It was disclosed that solutions of sucrose and the SAP could be mixed with an extruder. Then, the mixture was dried and ground into particles. The patent disclosed that with only 20% SAP (Sanwet) and 80% sucrose, they were able to obtain particles having a CRC value of 41.5 g/g.
U.S. Pat. No. 4,693,713 describes mixtures of a cross-linked SAP and monosaccharides or oligosaccharides. It was claimed that this mixture of water soluble compounds increased blood absorption. Generally, the SAP sugar ratio ranged from 3:1 to 5:1. It was mentioned that, polycarboxylic acids might be useful to cross link to with SAPs to CMC or CMS. This document does not disclose any other features of the cross linked SAP's obtained.
U.S. Pat. No. 6,765,042 discloses that CMC or CMS could be cross linked using a bifunctional cross linking agent and then dried and ground into particles that would have superabsorbent properties.
U.S. Patent Application Publication No. 2008177057 discloses that CMS particles prepared by reactive extrusion to form a gelatinized or pregelatinized mass and subsequently subjected to surface treatment in the presence of an acid and heat would form superabsorbent particles having useful FSC, CRC, and AUL values of at least 30, 20 and 15 in g/g respectively. It was disclosed that the acid may be a bifunctional group cross linking acid such as citric acid or succinic acid, or could be a non-crosslinking organic acid or even a non-crosslinking mineral acid such as hydrochloric acid, which appeared to cause formation of intramolecular cross links at the surface of the CMS particle. Godin et al. in WO App. 2010/096742 describe, similar results. While particles such as these exhibit FSC, CRC, and AUL properties similar to conventional polyacrylate SAPs, these CMS and CMC particles suffer from severe gel block problems which limits their use as a substitute for polyacrylate SAPs. Such particles demonstrate a SFR of less than 5-10 ml/min/g. By comparison, high tier polyacrylate derived SAPs usually have a SFR of more than 140 ml/min/g.
U.S. Patent Application Publication No. 2010/0057027 (WO 07/098932) discloses hypothetical superabsorbent particles made by combining polyacrylate materials and starch compounds, including CMS, pregelatinzed starches or other modified starches or combinations of different starches, where the maximum starch content is 30%, or 20% or 15% of the particle, the remainder being the polyacrylate material. The document disclosed generalized hypothetical combinations, whereby, powdery starch compounds may be mixed with the polyacrylate materials before, during, or after polymerization of the polyacrylate, or before, during or after drying of the polyacrylate, or before, during, or after a surface treatment of the polyacrylate with heat and cross linking agents to form cross links. The document proposed, cross linking the starch and the polyacrylate in the presence of one or more types of cross-linkers, a multi-arm cross-linker and a linear cross-linker. A hypothetical list of potential cross linkers includes diethylene glycol, triethylene glycol, polyethylene glycol glycerol polyglycerine, propylene glycol diethanolamine, triethanolamme, polyoxypropylene, oxyethyleneoxypropylene-blockcopolymers, sorbitan acid esters, polyoxyethylenesorbitan acid esters, trimethylolpropane, pentacrythritol, polyvinylalcohol, sorbitol, 1,3-dioxolan-2-one (ethylenecarbonate), 4-methyl-1,3-dioxolan-2-one (propylencarbonat), 4,5-dimethyl-i,3-dioxolan2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3dioxolan-2-one, 1,3dioxan-2-one, 4-methyl-i,3-dioxan-2-one, 4,6-dimethyl-i,3-dioxan-2-one. Surface treatment of the cross linked particles with either aluminum ions or oxazolidone is also proposed. The document fails to disclose and does not propose, any conditions for making a composite polyacrylate starch particle having a starch content of greater than 30% wt that would have a FSC, CRC, and AUL of greater than 40, 25, and 18 in g/g respectively, while at the same time retaining a SFR greater than 50 ml/mm or in the range of conventional SAP particles.
Fürno et al. in WO 2007/098932 describes graft-copolymers of carboxymethyl starches, however, the starch content in Fürno et al. was limited to 30% max. Moreover performances, especially in GBP (the near equivalent of SFR) were not as high as current SAPs.
Despite a long history of experimentation with combinations of starch and polyacrylate polymers for use as SAPs, there still is a need in the art to provide a SAP that has a high biobased content, and that has sufficient FSC, AUL and CRC properties in addition to sufficient SFR to be useful as a substitute for ordinary SAPs in absorbent products. The present disclosure addresses these problems and others, and provides further advantages that one of ordinary skill in the art will readily discern upon reading of the following non-restrictive description that follows.
The present disclosure refers to a number of documents, the content, of which is herein incorporated by reference in their entirety.